To determine whether the effects of oral contraceptives in increasing the cholesterol saturation of bile are mediated by increased secretion of cholesterol or by decreased secretion of bile acids and phospholipids, we measured the rate of biliary secretion of these lipids in 5 normal women during oral contraceptive usage and again during normal menstrual cycles on no medication. We also measured bile acid pool size and bile acid composition, as well as lipid composition and percent cholesterol saturation of gallbladder bile. These findings indicate that oral contraceptive usage increases biliary cholesterol secretion without changing bile acid pool size or bile acid composition, thereby raising the level of cholesterol saturation of bile and predisposing to cholesterol precipitation and gallstone formation.